It might be useful to know what kind of program it is, so that the notion of the names being "legal" can be understood better. So why does the original not just have the labels "last name" and "first name"?
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TimboFeb 26 '13 at 8:29

1

@Timbo: Because if you issue a legally binding document, it's not legally binding if the name (at the time) does not correspond with the name of your passport (at the time).
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QuandaryFeb 26 '13 at 8:34

If we use these termns in a user dialog it will be perfectly understood. In case we do need to put emphasis on the legal aspects (which is very uncommon in Germany when it comes to names) we may use the adjective "amtlich". This will include also that we had used these names for the registration office (Einwohnermeldeamt).

amtlicher Familiennamen

Note also that coming from the above the terms "Name", or "Nachnamen" are not used in official documents but they will be well understood as synonyms. The "frühere Namen", also known as "Geburtsnamen" when married people changed their name, the academic grade ("Doktorgrad"), or additional names ("Ordensnamen, Künstlernamen") are facultative.

I'm not sure you need this. There is no other concept in the German language than having the given name be your legal name. Quite the contrary: You must state explicitly if you want something else such as Künstlername (stage name) or Spitzname (nick name).

If you really want to emphasize that you want the real name you say just that: echter Name/Vorname (lit. real name/first name) but it does sound kinda clunky and redundant to me.

False name does not invalidate contract

I am not aware of any statute or case law that would invalidate a contract entered into under a false name. Order a big-screen TV set under an assumed name and you are still on the hook for the sale price.

It can be taken as given that when you are asked for forename / surname, it is the real name that is being requested. (A special case is if you are, e.g., an artist well-known under a different name, which may even be included on your officially issued ID.) As evidence for this contention I offer the following particulars:

The case of Heise Verlag

In 2006, German publisher Heise Verlag prevailed in a civil proceeding (DE) in Munich Regional Court. The court recognized the right of Heise -- the plaintiff in the proceeding -- to terminate the membership of a user of their online fora. The individual had registered under a false name. The court held that although the parties had entered into a binding agreement, the plaintiff was nonetheless entitled to terminate the defendant's membership for cause, based on his deception in giving a false name, besides defendant misbehaving in other ways.

If there were any doubt that when "Forename" and "Surname" are requested it is the real name that is being asked, Heise would surely have designed its online registration form (DE) accordingly. However, they ask only for "Forename" and "Surname" without additionally specifying that they be "legal" or "official". In their Terms of Use (DE), they do stipulate that registering under a false name is disallowed. However, this does not imply any ambiguity about whether a person's name is understood to be her or his real name.

Conclusion

As echter Name or amtlicher Name sound odd and may even confuse some people, I would advise talking to your project manager or client and recommending that they just write Vorname / Nachname, while noting in the "fine print" that use of a false name is prohibited.

Finally I would think that the foregoing is true not only for dealings in ordinary commerce but also for interactions with official bodies. The U.S. State Department's
Online Nonimmigrant Visa Application (DS-160) (EN) asks for "Surnames" and "Given Names" but does not specify "legal" or "official" on the data entry form. And the application form for a visa to Germany (DE, PDF) likewise asks only "Name (Surname)" and "Forename(s) (Middle Name(s))". Of course, both forms do note that applicants must enter only truthful information.